In recent years, devices, which handle image information as digital data, which, in such a case, aim to transmit and store information with a high efficiency, and which conform to a scheme such as MPEG (Moving Picture Experts Group), for compressing image information using orthogonal transformation, such as discrete cosine transformation, and using motion compensation by utilizing redundancy that is unique to the image information have become widespread in both information distribution in broadcasting stations and information reception in ordinary homes.
In particular, MPEG2 (International Organization for Standardization and International Electrotechnical Commission (ISO/IEC) 13818-2) is defined as a general-purpose image encoding scheme and is presently widely used in a wide range of applications for professional use and consumer use as standards for both interlaced-scanning images and sequential scanning images and standard and high-definition images. By employing the MPEG2 compression scheme, for example, a coding rate (bit rate) of 4 to 8 Mbps is allocated for an interlaced-scanning image of a standard resolution with 720×480 pixels, and a coding rate (bit rate) of 18 to 22 Mbps is allocated for an interlaced-scanning image of a high resolution with 1920×1088 pixels. As a result, a high compression ratio and a good image quality can be realized.
The MPEG2 has been mainly intended for high-image-quality encoding appropriate for broadcasting, but was not compatible with an encoding scheme for realizing a lower coding rate (bit rate) (a higher compression ratio) than that of MPEG1. With the popularity of mobile terminals, the demand for such an encoding scheme is expected to increase in the future. To respond this, standardization of MPEG4 encoding schemes have been confirmed. With regard to an image encoding scheme, the specification thereof was confirmed as the international standard ISO/IEC 14496-2 in December in 1998.
In addition, in recent years, originally for the purpose of video coding for television conferencing, standardization of specifications of a standard called H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)) has progressed. H.26L is known to achieve higher encoding efficiency although it requires a greater amount of computations for encoding and decoding than conventional encoding schemes such as MPEG2 and MPEG4. Moreover, currently, as part of the activity of MPEG4, standardization for incorporating functions, which are not supported by H.26L, into the H.26L is performed as Joint Model of Enhanced-Compression Video Coding to realize high encoding efficiency.
The schedule of standardization showed that, it became an international standard under the name of H.264 and MPEG-4 Part 10 (Advanced Video Coding, hereinafter referred to as AVC) in March, 2003.
However, setting the size of a macroblock to 16×16 pixels is not optimal for a large image frame named UHD (Ultra High Definition; 4000×2000 pixels) that will become an object of the next generation encoding scheme.
Thus, the standardization of an encoding system called HEVC (High Efficiency Video Coding) has been currently developed by JCTVC (Joint Collaboration Team-Video Coding), which is a joint standardization organization of ITU-T and ISO/IEC, for the purpose of further improving the encoding efficiency compared to AVC (for example, see Non-Patent Document 1).
In the HEVC encoding scheme, a coding unit (CU) is defined as the same processing unit as the macroblock in the AVC scheme. The size of CU is not fixed to 16×16 pixels unlike the macroblock of the AVC scheme but is designated in image compression information in respective sequences.
However, in order to improve encoding of motion vectors using median prediction defined in the AVC scheme, a method that allows “temporal predictor” and “spatio-temporal predictor” as well as “spatial predictor” to be used as candidates for predictive motion vectors has been taken into consideration (for example, see Non-Patent Document 2).
Moreover, a method called motion partition merging in which merge_flag and merge_left_flag are transmitted is proposed as one of encoding schemes for motion information (for example, see Non-Patent Document 3).